Shoot development in flowering plants is a continuous process ultimately controlled by the activity of the shoot apical meristem. Apical meristem activity during normal plant development is sequential and progressive, and can be summarized as a series of overlapping phases: vegetative→inflorescence→floral (V→I→F). Over the past 50 years many models have been proposed for the control of the vegetative-to-floral transition. These models range from simple single pathway models to complex multiple pathway models, and are largely based on physiological studies (for review, see Bernier, 1988). Modern techniques provide researchers with genetic and molecular methods that can be used to further investigate the control of V→I→F transitions.
One such modern technique now routinely practiced by plant molecular biologists is the production of transgenic plants carrying a heterologous gene sequence. Methods for incorporating an isolated gene sequence into an expression cassette, producing plant transformation vectors, and transforming many types of plants are well known. Examples of the production of transgenic plants having modified characteristics as a result of the introduction of a heterologous transgene include: U.S. Pat. No. 5,268,526 (modification of phytochrome expression in transgenic plants); U.S. Pat. No. 5,719,046 (production of herbicide resistant plants by introduction of bacterial dihydropteroate synthase gene); U.S. Pat. No. 5,231,020 (modification of flavenoids in plants); U.S. Pat. No. 5,583,021 (production of virus resistant plants); and U.S. Pat. Nos. 5,767,372 and 5,500,365 (production of insect resistant plants by introducing Bacillus thuringiensis genes).
Light quality, photoperiod, and temperature often act as important, and for some species essential, environmental cues for the initiation of flowering. However, there is very little information on the molecular mechanisms that directly regulate the developmental pathway from reception of the inductive light signal(s) to the onset of flowering and the initiation of floral meristems. The analysis of floral transition mutants in pea (Pisum sativum) (see Murfet, 1985) and Arabidopsis (see Koomneef et al., 1991) has demonstrated that at least part of the genetic hierarchy controlling flowering onset is responsive to the number of hours of light perceived by a plant within a 24 hour light/dark cycle. The monitoring of the length of the light period is referred to as the photoperiodic response. Photoperiodic responses have long been thought to be tied to one or more biological clocks that regulate many physiological and developmental processes on the basis of an endogenous circadian rhythm.
Many important physiological and developmental plant processes are influenced by circadian rhythms. These include the induction of gene transcription, leaf movement, stomatal opening, and the photoperiodic control of flowering. While the relationship of these plant processes to the circadian rhythm has long been recognized, the genetic analysis of circadian rhythms in plants has only recently begun. Most of the genetic analysis of circadian regulation has been performed with Drosophila and Neurospora crassa, where mutational studies have led to the isolation of the per and frq genes, respectively (Hall, 1990; Dunlap, 1993). These genes are thought to encode components of the circadian oscillator, in part because, while null alleles cause arrhythmic responses, alleles of these genes exist that produce either long or short period responses. Transcriptional production of per and frq mRNA cycles on a twenty-four hour period, and both genes regulate their own expression (Edery et al., 1994; Aronson et al., 1994).
Arabidopsis is a quantitative long-day (LD) plant—wild-type plants will initiate flowering more quickly when grown under LD light conditions than when grown under short-day (SD) light conditions. In order to identify genes required for floral initiation and development, populations of Arabidopsis thaliana ecotype Columbia grown in SD conditions have been screened for early-flowering mutants. Isolated mutants were then examined for additional shoot development anomalies, and those with discreet shoot phenotypes related to meristem function or light perception were considered for further analysis. Such mutants may identify genes that are part of functionally redundant pathways that operate, to varying degrees, as “fail-safe” mechanisms for ensuring shoot growth and reproductive development. Examples of such functionally redundant pathways have been described in studies of Drosophila (e.g., Hulskamp et al., 1990) and C. elegans (e.g., Lambie and Kimble, 1991). The key genes identified by these Arabidopsis screens were the TERMINAL FLOWER 1 (TFL1) gene and the EARLY-FLOWERING 3 (ELF3) gene (Shannon and Meeks-Wagner, 1991; Zagotta et al., 1992).
The early-flowering (elf3) mutant of Arabidopsis is insensitive to photoperiod with regard to floral initiation. Plants homozygous for a mutation in the ELF3 locus flower at the same time in LD and SD growth conditions, whereas floral initiation of wild-type plants is promoted by LD growth conditions (Zagotta et al., 1992; Zagotta et al., 1996). In LD conditions, the flowering time of the elf3-1 heterozygote is intermediate between wild-type and the homozygous mutant. In addition to being photoperiod-insensitive, all elf3 mutants display the long hypocotyl phenotype characteristic of plants defective in light reception or the transduction of light signals (Zagotta et al., 1992; Zagotta et al., 1996). The majority of long hypocotyl mutants that have been identified are defective in red light-mediated inhibition of hypocotyl elongation. In contrast, elf3 mutants are primarily defective in blue light-dependent inhibition of hypocotyl elongation, although they are also partially deficient in red light-dependent inhibition of hypocotyl elongation (Zagotta et al., 1996).
The availability of the ELF3 gene would facilitate the production of transgenic plants having altered circadian clock function and programmed photoperiodic responses. It is to such a gene that the present invention is directed.